This invention relates to a novel form of surge relief valve. Such valves are required in pipelines containing various fluids to control surging, but perhaps the most extreme requirements lie in the control of surging in high pressure crude oil pumped pipelines. Unless the surging is controlled in such lines the damage to the pipeline and its associated equipment can be environmentally catastrophic. Available anti-surge devices using line pressure to actuate them have characteristics which may under certain circumstances give rise on actuation to violent vibration and cavitation. Repeated actuation of such devices may result in damage to the devices themselves or to joints in the associated pipeline close to the device.